Synchronous Rectification Passes Muster for Military Applications

Nov. 9, 2005
For years, telecom and networking applications have benefited from the use of synchronous rectification and the high efficiency it brings to dc-dc conversion

For years, telecom and networking applications have benefited from the use of synchronous rectification and the high efficiency it brings to dc-dc conversion. However, in the high-reliability world of military and aerospace applications, synchronous dc-dc converters have not been commonly available. VPT, the Blacksburg, Va.-based provider of power supplies for mission-critical applications, hopes to change that situation with the recent introduction of its High Efficiency Reliability Optimized (HERO) Power System. Described as a new way to implement a dc-dc distributed power system, HERO consists of two new modules, the isolated DVHE dc-dc converter and the nonisolated DVPL, point-of-load converter. Both modules employ synchronous rectification to achieve high overall system efficiency.

According to Mike Bosmann, vice president of sales and marketing at VPT, three technical challenges have discouraged the use of synchronous rectification in high-rel dc-dc converters in the past. One was the challenge of getting MOSFET gate drive circuitry to operate over both the full input voltage range and the extended temperature encountered in military/aerospace applications. In addition, it should be noted that the full input voltage range also must take into account transient conditions.

According to Bosmann, a second challenge was making room in the design for the additional components that are required by the gate drive circuitry, which doesn’t exist in a nonsynchronous design. A related problem is the fact that semiconductor dies have not been shrinking as fast as packaged FETs and ICs. This is important because high-rel converters such as the DVPL and DVHE use hybrid construction, assembling bare die to a ceramic substrate. VPT overcame these challenges by developing a proprietary gate drive circuitry.

Operating with up to 91% efficiency, VPT’s isolated dc-dc converter (part number DVHE2800S) takes a 16-V to 40-V input (with 50-V transient) and steps it down to a 5-V output at up to 10 A. The converter comes in a hermetically sealed module that measures 2.125 in. × 1.125 in. × 0.417 in. The converter is based on a forward topology that uses magnetic feedback and switches at 260 kHz. It features remote sense, ±10% output trim range and undervoltage lockout. The converter offers no synchronization feature.

The DVPL POL converter operates with a typical efficiency of 95% in stepping down a 3.0-V to 5.5-V input to an adjustable output voltage. The output ranges from 0.8 V to 3.6 V for a 5-V input, or 0.8 V to 2.5 V for a 3.3-V input. The POL, which also features power sequency/tracking, measures 1.0 in. × 0.8 in. × 0.27 in.

Using the DVHE and DVPL modules, customers that previously relied on nonsynchronous isolated dc-dc converters and linear regulators to generate supply voltages can implement distributed power architectures much like those used in telecom. Although the input voltage is likely to be 28 V, rather than the 48-V nominal seen in telecom, the same configuration of using the isolated power converter to feed the nonisolated POLs can be employed. The result of this change is higher efficiency along with reductions in board space and weight—both of which are critical factors in aerospace designs.

Both the DVHE and DVPL feature hermetic packaging and are specified for operation over the military temperature range of -55°C to +125°C. This temperature range limits the field of components that can be used in building a power supply since some commercial ICs are note rated below -40°C. The DVHE and DVPL are designed to operate over the full military temperature range with no derating. In OEM quantities, pricing for the DVHE is $595 and $161 for the DVPL.

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